Exploring the photophysics and excited state reactivity of [Ru(4,4′-BTFMB)2 (L)]2+ complexes (L = bpy, phen, TAP) as photodynamic therapy agents: a theoretical investigation

In this work, we explore the excited states of three different [Ru(4,4′-BTFMB)₂ (L)]²⁺ complexes (C1: L = bpy (2,2′-bipyridine); C2: L = phen (1,10-phenantroline); and C3: L = TAP (1,4,5,8-tetraazaphenanthrene)), aiming to investigate the ligand effects on their photophysical and photochemical properties and also to evaluate their suitability as photosensitizers (PS) for photodynamic therapy (PDT). Compound C3 is a new theoretical proposition for which we have observed a significant lowering in the HOMO and LUMO energies, which can be interpreted as an extra stabilization of the complex, a highly desirable feature for a PS candidate. Also, the low-lying excited states showed a shift in the MLCT transition nature from to , confirming that the presence of a more electronegative atom in the third ligand lowers the energy of the orbitals due to a more effective π-backbonding. Furthermore, the investigation of the excited state reactions has shown a dual character possibility for the three complexes, being thermodynamically favorable for photosensitizing molecular oxygen through energy transfer and oxidizing guanosine-monophosphate through electron transfer. Complex C3 presented the best reaction free-energy profile, indicating that this new compound is most suitable for use as a photosensitizer in PDT.